Lesson Video: The Human Genome Biology • First Year of Secondary School

Video Transcript

In this video, we’ll learn about the components of the human genome. First, we’ll learn about how our genetic information is organized on DNA. Then, we’ll outline the scientific adventure to unravel the secrets of DNA and our genome. And finally, we’ll see how these discoveries have fundamentally changed our understanding of life science, medicine, and evolution. So, let’s dive in and learn more about the human genome.

The genome is all the genetic information of an organism and is necessary to make me, you, and this little figure shown here. Most of the cells in our body contain the genome inside the nucleus as DNA. If we were to take out all the DNA from the nucleus of one of our cells and stretch it out, we would have about two meters worth of DNA. These two meters of DNA are actually organized into 46 pieces that are called chromosomes. Half of these 46 chromosomes come from the sperm cell from our biological father, while the other half come from the egg cell from our biological mother. When the egg is fertilized by the sperm cell, an embryo develops with 46 chromosomes. This is why we end up having 46 chromosomes in the cells of our body.

In order to study our chromosomes, we can arrange them using a karyotype. In a karyotype, our chromosomes are numbered from one to 22. This is based on size, so chromosome one is bigger than chromosome 22. We have a pair of each chromosome because one comes from our biological father and the other comes from our biological mother, as we have just seen. The 23rd pair of chromosomes are called the sex chromosomes because they determine our biological sex. Having a single copy of both X and Y chromosomes makes a biological male, while having two copies of the X chromosome makes a biological female.

Now, let’s take a closer look at one of these chromosome pairs, for example, chromosome number nine. As we’ll see, chromosomes in a pair are not exactly the same. They don’t contain the exact same information. So, we’ll first need to untangle the DNA that makes up these chromosomes. Our chromosomes are made of chromatin, which is made of the DNA coiling around proteins called histones that help to compact and protect our DNA. If we can uncoil this condensed chromatin and zoom in, we would see that the DNA has a double-helix shape. We will come back to this structure in a few moments. Some sections in the chromosomes contain the information that tells the cell how to produce certain proteins or other functional units. We call these sections of DNA genes.

Let’s pretend that the section of DNA on our chromosome nine pair codes for an eye protein that determines if our eyes are blue or green. Two different versions of the same gene are called alleles. So, for example, our biological father could’ve passed on the blue eye allele, while our biological mother could’ve passed on the green eye allele. Specific laws of genetics then determine for each gene how our parental alleles are expressed. There’s a large number of genes on our 46 chromosomes that will influence each of our body’s characteristics like eye color, for example. This is why we all end up looking like a mix of our parents.

Now that we understand more about how our DNA is organized and inherited, let’s learn about its molecular structure. So, here’s a section of DNA, and as we see, it contains two strands. In between these strands are the different base pairs. Let’s zoom in so we can get a closer look. DNA is made up of repeating subunits called nucleotides; here, we’re just looking at one of them. Each nucleotide contains a phosphate group, a sugar, and a nitrogenous base, often simply called a base, that could be either guanine, or G for short, shown here in orange; cytosine, as shown in blue; adenine, as shown in green; and thymine, as shown in pink. These nucleotides can be joined together to form a polymer that makes up a single strand of DNA. We call the structure in black the sugar phosphate backbone.

If we now look at the second strand, we can see how these individual nucleotides compare with each other. This is accomplished by complementary base pairing, where one nitrogenous base pairs with another. Guanine always pairs with cytosine, and adenine always pairs with thymine. These base pairs are joined due to hydrogen bonding. There are three hydrogen bonds between guanine and cytosine, while there are two between adenine and thymine. These hydrogen bonds are what hold the two strands of DNA together and help to form this double-helix structure. These different bases in the nucleotides of DNA can form a specific sequence that can provide the information for a particular gene. So, if we look back towards the diagram on the left and we wanted to determine the sequence of this strand, we would see that it’s GTT, TGA, and GGA.

The Human Genome Project was a huge scientific research project which aimed to determine the sequence of base pairs that make up human DNA. But it wouldn’t have been possible without some other key moments in DNA’s history. Let’s start from when the double-helix structure of DNA was discovered.

The discovery of DNA structure was a turning point in the history of science. The discovery was made in 1953 by James Watson and Francis Crick, with key evidence for the double-helix structure provided by Rosalind Franklin. Then, scientists continued to unravel the rules that determine how the DNA information is translated in the proteins. This is called the genetic code, and it was cracked in 1966. In 1975, the first DNA sequencing method was developed to determine the sequence of bases in the DNA molecule. Using this method, it was then possible to determine the sequence of a gene causing the genetic disorder Huntington’s disease. By doing so, we were able to map this gene or determine its position in the human genome. This showed us how useful it would be for medicine to sequence and map all the genes in a human genome. This is why an international collaboration of scientists called the Human Genome Project was launched in 1990 to accomplish this huge task.

In 2003, scientists revealed the first complete map of genes and their sequence in a reference human genome. There were still gaps between the genes and various regions of the genome that were more difficult to sequence. And it’s really in 2021 that the entire sequence of the human genome has been totally finished. Now, let’s look at some of the outcomes of the Human Genome Project. It took 13 years for scientists to provide a sequence and a map of all the human genes contained in 23 chromosomes. In all, this represents about three billion nucleotides. This is a huge number. If it took you one second to say each nucleotide, it would take around 100 years to say the entire sequence of the human genome. So, what else have we gained from this project?

First, by working on the project, scientists have improved their sequencing methods, analysis, and costs. It’s now possible to sequence an entire human genome in just one day for a fraction of the old method costs. Second, the human genome has helped to advance many fields of medicine and biomedical research, for example, to screen genetic disorders and genes contributing to some cancers. Sequencing the genome of cells can also help doctors to make diagnoses and offer more adapted treatments. The Human Genome Project has also helped us to understand more about how our species has evolved. Now that we understand a bit more about the human genome, let’s take a moment to try out a practice question.

Which of the following statements about human chromosomes is true? (A) Each chromosome in a somatic, also known as a body, cell contains the entire human genome. (B) All chromosomes are the same size and contain the same number of genes. (C) Somatic, also known as body, cells contain half the number of chromosomes that a gamete or sex cell contains. Or (D) a single chromosome can carry hundreds to thousands of genes.

Three of these four statements are false, and we’re looking for the one that’s true regarding human chromosomes. So, what is a chromosome exactly? Each human body cell or somatic cell contains all the genetic information that’s necessary to make a human. This genetic information is stored in the nucleus of cells in the form of compacted DNA. This molecule is not continuous though. In humans, it’s actually segmented into 46 pieces called chromosomes. These 46 chromosomes represent all the genetic information in our cells, which we call the human genome. So, we can already eliminate this statement because the entire genome is contained not in a single chromosome, but in a set of these 46 chromosomes.

These 46 chromosomes are arranged into 23 pairs of chromosomes. Chromosomes one to 22 are arranged based on their size, with chromosome one being the largest and chromosome 22 being the smallest. The 23rd pair of chromosomes, made up of chromosomes X and Y, represent the sex chromosomes, which are responsible for determining the biological sex of the person. Chromosomes within a pair carry a very similar number of genes. Genes are sections of chromosomes that code for a functional unit, such as a protein. The largest of them, chromosome one, carries just over 2000 genes, while chromosome 22, the smallest, contains about 500 genes. We can then eliminate this answer choice because all chromosomes aren’t necessarily the same size or contain the same number of genes.

Besides body or somatic cells, the human body can also produce sex cells. These sex cells, also called gametes, do not contain 46 chromosomes. Instead, they only contain one chromosome from each pair, so just 23 in total. This is so that when the sex cells combine during fertilization, the resulting embryo has the correct number of chromosomes in each of their cells, or 46 in total. So, this statement is also incorrect because it’s the gametes that contain half the number of chromosomes, not the somatic cells. This then leaves this statement, which is true because chromosomes can contain hundreds to thousands of genes.

Now, let’s take a moment to go over the key points that we covered in this video. The human genome is all the genetic information contained within a human cell. The human genome is made up of DNA, which is made of nucleotides that form a double helix that’s compacted into chromosomes. There are 46 chromosomes in most human cells. Genes are sections of DNA that code for functional units, for example, proteins. There can be different versions of genes that are called alleles. The Human Genome Project was a massive international project that aimed to sequence the human genome and map all the different genes contained within. Some of the major outcomes of the Human Genome Project include advances in medicine and a better understanding of human evolution.